<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8" />
    <title>Document</title>
    <script src='lib/webgl-utils.js'></script>
    <script src='lib/webgl-debug.js'></script>
    <script src='lib/cuon-utils.js'></script>
</head>
<body onload="main()">
<canvas id="mybox" width="500" height="500">
    请更换新版浏览器查看效果!
</canvas>
<script >
   let VSHADER_SOURCE=`
   attribute vec4 a_position;
   void main(){
    gl_Position=a_position;
    }`;
   let FSHADER_SOURCE=`
   void main(){
    gl_FragColor=vec4(1.0,1.0,0.0,1.0);
    }`;

    function main(){
        let canvas=document.getElementById('mybox');
        let gl=getWebGLContext(canvas);
        if(!initShaders(gl,VSHADER_SOURCE,FSHADER_SOURCE)){
            console.log('初始化shader失败');
            return;
        }
        let n=initVertexBuffers(gl);
        if(n<0){
            console.log('设置顶点位置失败');
            return;
        }
        gl.clearColor(0.0,0.0,0.0,1.0);
        gl.clear(gl.COLOR_BUFFER_BIT);
        gl.drawArrays(gl.TRIANGLE_STRIP,0,n);
        // gl.drawArrays(gl.TRIANGLE_FAN,0,n);


    }

    function initVertexBuffers(gl){
        let vertices=new Float32Array([-0.5,0.5,  -0.5,-0.5,  0.5,0.5,  0.5,-0.5]);
        let n=4;
        let vertexBuffer=gl.createBuffer();
        gl.bindBuffer(gl.ARRAY_BUFFER,vertexBuffer);
        gl.bufferData(gl.ARRAY_BUFFER,vertices,gl.STATIC_DRAW);
        let a_position=gl.getAttribLocation(gl.program,'a_position');
        if(a_position<0){
            console.log('a_position 不存在');
            return ;
        }
        // 将缓冲去变量分配给attribute变量
        gl.vertexAttribPointer(a_position,2,gl.FLOAT,false,0,0);
        // 开启attribute变量
        gl.enableVertexAttribArray(a_position);
        return n;
    }

</script>

</body>
</html>